1. Field of the Invention
The present invention relates to an improvement in a method for preparing a superconducting thin film. More particularly, it relates to a method for prearing a superconducting thin film of compound oxide having a high critical temperature and improved uniformity in composition and also possessing lasting stability for a long period.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a phenomenon of a kind of phase change of electrons under which the electrical resistance become zero and the perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power.
Superconducting devices are well-known in the field of electronics. Typical superconducting device is so called Josephson device in which quantum efficiency is observed macroscopically under the Josephson effect when an electric current is passed through a weak junction arranged between two sauperconducting bodies.
Tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
However, the critical temperature of the conventional superconducting material could not exceed 23.2.degree. K. of Nb.sub.3 Ge which was the highest Tc for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovrered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. Japanese patent laid-open No. 56-109824 discloses a method for preparing a thin film of superconductor represented by BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 0.05.ltoreq.x.ltoreq.0.35). In this patent, the thin film is prepared by high-frequency sputtering technique which is carried out in an oxygen containing atmosphere and then the resulting film is further heated at 500.degree. to 550.degree. C. These superconductors, however, possess rather lower transition temperatures of about 10.degree. K. and hence usage of liquidized helium (boiling point of 4.2.degree. K.) as cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La,Sr].sub.2 CuO.sub.4 which is called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type oxides shown such higher Tc as 30.degree. K. which are extremely higher than known superconducting materials.
It was also reported in the news parer that C. W. Chu et al discovered in the United States of America another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of in the order of 90.degree. K. in Feb. 1987. Still another type new superconducting material is a compound oxide of Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system which exhibit such high Tc as more than 100.degree. K. and which are chemically much stable than the abovementioned YBCO type compound oxide or the like.
And hence, possibility of existence of high-temperature superconductors have burst on the scene.
A thin film of superconducting compound oxide may be prepared by sputtering, ion-plating, vacuum deposition, molecular beam epitaxial process or the like. For example, a thin film of the abovementioned YBCO type is prepared by sputtering technique in which a sintered body composed of YBa.sub.2 Cu.sub.3 O.sub.7-x is used as a target. The sputtering conditions for preparing superconducting thin films of compound oxide are disclosed also in our co-pending U.S. patent application Ser. No. 152,714 filed on May 2, 1988.
The abovementioned sputtering technique itself is satisfactory but some of the thin films obtained by such physical vapour deposition technique do not or hardly possess the superconducting property. This fact is explained by defectiveness of oxygen deficiency, in other words, imperfect oxygen contents in the crystal. In fact, the superconductivity or superconducting property of the abovementioned new type superconducting material of compound oxide is influenced by the oxygen contents or oxygen deficiency in the crystalline structure. And hence, if the oxygen deficiency is not proper, neither high-Tc nor reduction of discrepancy between the critical temperature and onset temperature can be realized.
In order to overcome such drawback, it is a usual practice that the deposited thin film prepared by the physical vapour deposition technique is after-treated or annealed in the presence of oxygen gas under heat for example at a temperature ranging from 700.degree. to 1,000.degree. C. This heat-treatment convert the incomplete oxygen deficiency to the proper value that is suitable for realizing high-Tc (critical temperature) and high-Jc (critical current density).
In prior art, the heat-treatment is carried out in a furnace which is equipped specially for this end. In other words, a substrate on which a superconducting thin film is deposited is taken out of a sputtering machine to set the substrate in the furnace. In this operation, however, the substrate having the superconducting thin film thereon is necessarily cooled down and is exposed to surrounding atmosphere and hence desired control of atmosphere can not be assured, so that the resulting products do not possess uniform property and are deteriorated with the passing of time.
Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a method for prearing a superconducting thin film improved in stability during storage or during use.